It is highly desirable in order to have consistent high quality soldering that all soldering parameters be held as uniform as possible. In particular, it is desirable that the composition of the solder composition itself remain within specific ratios. However, under the conditions normally encountered in commercial mass soldering processes it is extremely difficult, if not impossible, to prevent metallic impurities from being introduced into the molten solder bath. For example, during the mass wave soldering of electronic circuit boards and the like, the molten solder bath is normally contaminated by impurities leached from the circuit boards which are being soldered.
Certain of these metallic impurities, even when present in relatively small amounts, can dramatically and adversely affect the wetting and alloying properties of solder compositions. Certain of the metallic impurities may promote the formation of inter-metallic compounds which are insoluble in the solder composition, resulting in solder joints having a gritty consistency. Other metallic impurities readily oxidize at the surface of the solder bath, thereby increasing the surface tension of the solder which changes the solder flow characteristics. The presence of impurities is also known to give rise to defects in the finish soldered product by causing undesired bridging between closely spaced components and circuitry and also by increasing the tendency for the formation of solder icicles on the soldered circuit boards.
It is accordingly important to carefully monitor both the type and the amount of impurities in the solder. This is now done by chemical analysis, spectrographic analysis or by various other techniques, all of which are expensive to conduct and often are extremely time-consuming. In a dynamic manufacturing situation where the soldering is continued while tests are being run, the actual solder composition may well vary substantially from the analyzed results by the time the results of the tests are obtained.
In copending patent application of Tosima et al., Ser. No. 327,595, filed Dec. 4, 1981, U.S. Pat. No. 4,409,333, entitled Method for Evaluating Solderability, a method is disclosed for the determination of the presence and amount of colored metals, such as copper and gold, present as impurities in silvery white solder baths. The Tosima et al. method utilizes as a test piece a transparent substrate which has a layer of the colored metal on one surface of the substrate. The surface of the substrate having the colored metal layer is immersed in the solder alloy bath. The colored metal layer on the substrate forms a silvery white copper alloy with the solder and the color change is observed through the substrate. The time required for the color change is indirectly related to the relative amount of the colored metal present as an impurity in the solder bath.
The Tosima et al. method is highly accurate, fast and effective for determining the amount of colored metal impurities such as copper or gold in a silvery colored solder composition, but is not effective for determining the presence and the relative amount of silvery white metal impurities such as silver, nickel, zinc, and bismuth in silvery white solder compositions.
What would be highly advantageous would be an optical evaluation method similar to that used for the colored metal contaminates which could be used to determine the presence and amount of silvery white metal impurities in silvery white solder compositions which would be both simple to conduct and would give rapid and accurate results.